In a mobile terminal such as a cellular phone or a mobile game console, a liquid crystal display device is generally used as a display means. Since a cellular phone is driven by a battery, a power consumption is strongly required to be reduced. For this reason, information such as time or a battery life, which is required to be always displayed, is displayed on a reflective sub-panel. In recent years, on the same main panel, a normal display by a full-color display and a reflective always-on display have been required to be compatible.
FIG. 26 shows an equivalent circuit of a pixel circuit in a general active-matrix type liquid crystal display device. FIG. 27 shows an example of a circuit arrangement of an active-matrix type liquid crystal display device having m×n pixels. Both reference symbols m and n denote integers each of which is 2 or more.
As shown in FIG. 27, switch elements configured by thin film transistors (TFTs) are arranged at intersections between m source lines SL1, SL2, . . . , SLm and n scanning lines GL1, GL2, . . . , GLn. In FIG. 26, the source lines SL1, SL2, . . . , SLm are represented by a source line SL, and, similarly, the scanning lines GL1, GL2, . . . , GLn are represented by a symbol GL.
As shown in FIG. 26, a liquid crystal capacitor element Clc and an auxiliary capacitor element Cs are connected in parallel to each other through a TFT. The liquid crystal capacitor element Clc is configured by a laminated structure in which a liquid crystal layer is formed between a pixel electrode 20 and a counter electrode 80. The counter electrode is also called a common electrode.
FIG. 27 simply shows only a TFT and a pixel electrode (black rectangular portion) in each pixel circuit.
The auxiliary capacitor element Cs has one end (one electrode) connected to the pixel electrode 20 and the other end (other electrode) connected to an auxiliary capacitive line CSL to stabilize a voltage of pixel data held in the pixel electrode 20. The auxiliary capacitor element Cs advantageously suppresses a voltage of pixel data held in a pixel electrode from varying due to generation of a leakage current in the TFT, a variation in electric capacity of the liquid crystal capacitor element Clc between a black display and a white display caused by dielectric anisotropy held by liquid crystal molecules, a variation in voltage through a parasitic capacity between a pixel electrode and a peripheral wire, and the like. Voltages of the scanning lines are sequentially controlled to turn on TFTs connected to one scanning line, and voltages of pixel data supplied to source lines are written in corresponding pixel electrodes, respectively, in units of scanning lines.
In a normal display by a full-color display, even though display contents are a still image, the same display contents are repeatedly written in the same pixel for each frame. In this manner, the voltages of the pixel data held in the pixel electrodes are updated to minimize a variation in voltage of the pixel data and to secure a display of a high-quality still image.
A power consumption to drive a liquid crystal display device is almost controlled by a power consumption to drive a source line by a source driver, and is almost expressed by a relational expression represented by the following numerical expression 1. In numerical expression 1, reference symbol P denotes a power consumption; f, a refresh rate (the number of times of a refresh action of one frame per unit time); C, a load capacity driven by a source driver; V, a drive voltage of the source driver; n, the number of scanning lines; and m, the number of source lines. In this case, the refresh action is an operation that applies a voltage to a pixel electrode through a source line while keeping display contents.P∝f·C·V2·n·m  (Numerical Expression 1)
In the always-on display, since the display contents are a still image, the voltage of the pixel data needs not be always updated for each frame. For this reason, in order to further reduce the power consumption of the liquid crystal display device, a refresh frequency in the always-on display state is lowered. However, when the refresh frequency is lowered, a pixel data voltage held in a pixel electrode varies by a leakage current of a TFT. The variation in voltage causes a variation in display luminance (transmittance of liquid crystal) of each pixel and becomes to be observed as flickers. Since an average potential in each frame period also decreases, deterioration of display quality such as insufficient contrast may be possibly caused.
In this case, as a method of simultaneously realizing a solution of a problem of deterioration of display quality caused by a decrease in refresh frequency and a reduction in power consumption in an always-on display of a still image such as a display of a battery life or time, for example, a configuration described in the following Patent Document 1 is disclosed. In the configuration disclosed in Patent Document 1, liquid crystal displays by both transmissive and reflective functions are possible. Furthermore, a memory unit is arranged in a pixel circuit in a pixel area in which a reflective liquid crystal display can be obtained. The memory unit holds information to be displayed in a reflective liquid crystal display unit as a voltage signal. In a reflective liquid crystal display state, a voltage held in the memory unit of the pixel circuit is read to display information corresponding to the voltage.
In Patent Document 1, the memory unit is configured by an SRAM, and the voltage signal is statically held. For this reason, a refresh action is not required, and maintenance of display quality and a reduction in power consumption can be simultaneously realized.    Patent Document 1: Unexamined Japanese Patent Publication No. 2007-334224